A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In an immersion lithographic apparatus, immersion fluid is handled by a fluid handling system or apparatus. In an embodiment the fluid handling system or apparatus may supply immersion fluid and therefore comprise or consist of a fluid supply system or apparatus. In an embodiment the fluid handling system or apparatus may at least partly confine immersion fluid. In an embodiment the fluid handling system or apparatus may provide a barrier to immersion fluid and thereby comprise or consist of a barrier member, such as a fluid confinement structure. In an embodiment the fluid handling system or apparatus may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the fluid handling system or apparatus may be referred to as a seal member; such a seal member may be a fluid confinement structure. In an embodiment, immersion liquid is used as the immersion fluid. In that case the fluid handling system or apparatus may be a liquid handling system or apparatus. In the following description, reference to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.
In some immersion lithographic apparatus, there is a gap between the liquid confinement structure and the final element of the projection system (sometimes referred to as the WELLE lens). A free meniscus of the immersion liquid may be located in the gap. Typically, gas flows through the apparatus for thermal conditioning. Some of the gas stream may flow into the gap, over the liquid confinement structure. The gas stream may disadvantageously cause the immersion liquid to evaporate, thereby applying a thermal load on the liquid confinement structure and the projection system. The thermal load may cause thermal (e.g., cold) spots on the projection system. Depending on the location of the meniscus, the thermal spots may cause optical aberrations and/or may contribute to overlay/focus irregularity. In addition, some of the immersion liquid may escape from the liquid confinement structure by passing through the gap.
During exposure, the substrate table is moved relative to the liquid confinement structure (and the projection system). The movement may cause the immersion liquid within the liquid confinement structure to move up the gap, e.g., in the direction of travel of the substrate. When the substrate is moved through a scanning or stepping motion, the direction of movement of the substrate changes. Because the immersion liquid in the gap moves with the movement of the substrate, the movement causes the level of the immersion liquid in the gap to change. The movement of the immersion liquid may be referred to as sloshing. If the movement is sufficiently large, the pressure on the immersion liquid may be sufficient to cause the liquid to overflow onto the top surface of the liquid confinement structure. When the pressure decreases, liquid flows back into the gap, possibly taking with it unwanted contaminating particles present on the surface of the liquid confinement structure. Liquid may also be left on the top surface of the liquid confinement structure, and on a surface of the projection system. The remaining liquid may subsequently evaporate into the surrounding gas and thus apply a thermal load to the respective surfaces.
A material that is liquidphobic with respect to the immersion liquid may be provided on an external surface of the projection system in the region of the gap. During sloshing, the liquidphobic material can help prevent the immersion liquid from moving too far upwards or outwards along the gap. The liquidphobic material may reduce the amount of liquid that is left behind on the projection system when the liquid level recedes. The liquidphobic material may shape the meniscus so as to reduce heat transfer from the projection system. The liquidphobic material may be applied as a coating or as a sticker.
It is an object of the invention to provide apparatus and methods for providing a material on an external surface of the projection system in an effective manner.